Population "is hugely important," to the economic and environmental health of the world in coming decades, says Carl Haub, a demographer at the Population Reference Bureau in Washington.

Most economists see a significant range of benefits:

* Poor nations would find it easier to raise living standards.

*The possibility of famine would be reduced, especially in Africa and Asia.

* Developing nations would become thriving markets for US exports.

* Immigration pressures on the US and other rich countries might ease.

And environmentalists, afraid of severe damage to the world's natural resources from so many people, would breathe a sigh of relief.

Most industrial countries already have birth rates that, if sustained, will lead in the years ahead to declining populations. But population growth is still brisk in most developing nations. Some 98 percent of the increase in coming decades will take place in India, Pakistan, Nigeria, Egypt, and other developing countries.

Problems with too many people

Rapid population growth makes it more difficult for developing nations to improve education and health systems, protect the environment, and provide enough jobs and food. Parents with many children find it hard to set aside savings that can be used to advance productivity.

Low population growth, says Mr. Haub, will make it "infinitely easier" to manage social and economic challenges.

The difficulty in making long-term population projections is that they rely on decisions not yet made: How many children will future parents decide to have?

"Guessing about long-range future population trends," says Nicholas Eberstadt, a demographer at the American Enterprise Institute in Washington, "is a mug's game." Speculation of a depopulating world arises from a larger than expected decline in fertility.

"Probably most people in this field are a lot more hopeful [for manageable population growth] than 10 or 20 years ago now that we have evidence that family planning programs are making a difference," says Ward Rinehart, project director of the Population Information Program at Johns Hopkins University in Maryland

"But I'm not ready to say the population problem is solved," he adds. "The message is, let's keep going to help people achieve their fertility goals."

In its biennial count, the UN estimated last year that the world population stood at 5.77 billion, 29 million less than predicted in 1994.

The reason was a drop in fertility to a world average of 2.96 children per woman instead of the anticipated 3.1. Lower fertility in populous China, Indonesia, and Brazil was especially significant.

This "total fertility rate" peaked in the late 1950s at close to five children per woman. Population would stabilize after some decades if fertility dropped to 2.1 children per woman.

Lower growth rates

The world's annual population growth rate peaked at a rapid 2.04 percent in the late 1960s. In 1990-95, it fell to 1.48 percent a year.

A third peak in the number of people added to the world population arrived in 1985-1990 at 87 million annually. Last year, "only" 81 million were added - roughly the population of Germany.

In the history of the human race, these peaks are "monumental events," says Joseph Chamie, director of the UN's Population Division. "We saw something that had never been seen before."

The UN projection used most often sees the fertility rate declining to 2.1 in 2050 when the world's population would reach 9.4 billion. That's nearly half a billion below the estimate two years ago. Population would stabilize at 10.7 billion in 2120.

Stan Becker, a demographer at Johns Hopkins, says he's a "pessimist" and doubts the accuracy of the UN's low variant.

"Before population peaks," he predicts, "there will be a disaster here and there" - such as natural disasters and famines in some poor, badly managed nations.

Those less fearful of famine count on a continued "green revolution" - the development of higher-yield grains and other foods - to provide adequate nutrition. Yields for rice, wheat, and corn have risen enormously.